Metal-air batteries, such as zinc-air batteries, offer the advantage of very high energy densities (up to 300 Wh/kg) over known conventional batteries, like lead-acid batteries, used to power electric vehicles. This is possible because, unlike a conventional battery cell that is comprised of two metal electrodes, a metal-air battery cell has only one metal electrode and a light-weight air cathode that absorbs air. For example, in a zinc-air cell, oxygen in the air is converted to hydroxyl ions, which oxidize the zinc anode, and water and electrons are released to produce electricity. The high energy density of metal-air batteries, like zinc-air batteries, translates into long operating range for electric vehicles, which in combination with low commercial production costs and a high degree of safety for both the environment and the consumer, offer significant advantages over conventional batteries for use in large consumer applications, like electric vehicles.
Experimental rechargeable metal-air batteries, like zinc-air batteries, have been built for use in electric vehicles and these batteries use a water-based electrolyte to convert oxygen to hydroxyl ions, which react with the zinc, to produce electricity. Because the air cathode of a metal-air cell passes water molecules as easily as oxygen molecules (due to similar molecular size and polarization), water loss is often experienced from the electrolyte if the ambient humidity is less than the equilibrium relative humidity value for the metal-air cell. This drying out of the cell may cause failure. Additionally, heat produced by the electrolytic reaction tends to increase water loss from the cell.
Batteries are sized to match the application in which the particular battery will be used. High-power applications, like powering traction motors in electric vehicles, tend to use large batteries including hundreds of individual metal-air cells electrically connected within the battery. Smaller batteries such as those used in consumer electronic devices can often use smaller batteries having fewer metal-air cells. The larger the battery, the more heat the battery will produce in operation. When larger quantities of heat are generated, more water evaporates from the electrolyte within the battery. Consequently, the electrolyte often must be replenished, especially in larger batteries, or the battery may fail. An automatic system to monitor cell performance and to add electrolyte to the battery when needed is desired in order to make larger batteries, such as traction batteries, easier to maintain and operate.
In addition to water loss from the electrolyte, there are other problems associated with electrolyte that interfere with performance of a metal-air battery. Carbonation of the electrolyte, due to a reaction of carbon dioxide with certain cell components and the electrolyte, interferes with the electrochemical reaction. In a zinc-air battery, uneven distribution of the electrolyte near the zinc anode, resulting in local concentration gradients of electrolyte, contribute to dendrites of zinc growing from the zinc anode to the air cathode during cycling of the cell. Eventually, dendrite formation may cause the cell to short out. Additionally, leakage of excess electrolyte can cause cell failure and corrosion of cell surroundings.
External replenishment methods and systems for batteries with a limited number of cells are known, wherein electrolyte is manually added to a common solution tank and is dispensed to the cells via ports and/or ducts under vacuum-induced pressure. U.S. Pat. No. 3,483,042 to Hulse, U.S. Pat. No. 3,630,786 to Ibaraki, et al., and U.S. Pat. No. 3,892,595 to Bell, et al. disclose such one-time manual methods and devices for filling battery cells with electrolyte.
U.S. Pat. No. 4,702,972 to Matsumoto discloses an electrolyte replenishing system specifically for a laminated type fuel cell wherein excess electrolyte is collected and recycled by means of a pump. Matsumoto '972 provides a system for continuous replenishment of electrolyte, but is specifically designed, for use with a laminated type fuel cell and is not automated to provide specific amounts of electrolyte at specific time intervals.
Therefore, a distributing system is desired that can satisfactorily replenish water and/or electrolyte loss experienced by a battery used in large consumer applications, like electric vehicles, and can effectively control electrolyte levels within the battery, such that only enough electrolyte as is needed for operation of the battery is provided on a periodic, automatic basis.